4 BTJLLETII7 805, U. S. DEPARTMEI^T OF AGRICULTURE. 



CHARACTER OF INJURY. 



The injury caused by the apple leafhopper to nursery apple trees 

 is due to the feeding of the nymphs and adults on the underside of 

 the tender terminal leaves from which they extract the plant juices. 

 As a result of this attack the leaves become undersized and curled 

 (PI. I, B), causing a decided check to the growth of the new wood. 

 The curling begins at the apex and extends toward the base of the 

 eaves, the lower surface always being rolled in. This type of injury 

 differs from aphis leaf-curl in that aphids roll the leaves more tightly 

 and curl them from the sides instead of from the tips. During the 

 progress of the injury produced by Empoasca mali the leaves become 

 wrinkled and the loss of sap finally causes the tips to dry up and turn 

 brown. (See PI. II, fig. 2.) 



The nymphs, because of their greater numbers and due to the fact 

 that they spend the entire nymphal period on a few leaves only, cause 

 more serious injury than do the adults. The latter feed only for a 

 short time, being principally engaged in egg-laying, and during this 

 period they fly from one tree to another. Injury by the feeding of the 

 adults, therefore, is of little importance when compared with the local- 

 ized injury produced by the nymphs. Consequently, the stunted ter- 

 minal growth is most apparent at the time when the nymphs are 

 most abundant on the foliage. As the nymphs gradually disappear 

 the terminal shoots seem to revive and develop nt)rmal leaves above 

 the stunted ones. (See PI. I, A.) At the time of infestation by the 

 next brood of nymphs, however, a similar check to the new terminal 

 growth is produced. Thus retardation in growth occurs periodically 

 throughout the season corresponding to the periods of infestation by 

 the successive nymphal broods, while intervening between each infes- 

 tation there is a short period during which the terminals maintain 

 a normal growth. Although the different broods of nymphs over- 

 lap slightly the successive checks in terminal growth usually are well 

 defined. 



In the vicinity of West Chester, Pa., there are three broods during 

 the season and three corresponding checks in the terminal growth. 

 The first growth-check takes place during the latter part of June 

 when the first nymphal brood is feeding; a second and a third check 

 appear during the latter part of July and August, respectively, at 

 the time when the second and third broods of nymphs are most 

 active on the foliage. The first brood is the most abundant on apple 

 and consequently causes more injury than do either of the two fol- 

 lowing broods. Adults of the first brood do not confine their activ- 

 ities to apple alone, as many scatter to other host plants to feed and 

 oviposit. 



